(a) Technical Field
The present invention relates to a lean NOx trap (LNT) catalyst with enhanced NOx storage capacity at low temperature. In particular, the present invention relates to an LNT catalyst with enhanced NOx storage capacity at low temperature and with significantly inhibited thermal desorption, which includes a first catalyst powder in which barium (Ba) and a precious metal are supported on a ceria support and a second catalyst powder in which a precious metal is supported on a magnesium (Mg)-substituted alumina support. More particularly, the first and second catalyst powders are provided as a washcoat which is coated on a honeycomb-type carrier, followed by and drying and baking to form the LNT catalyst.
(b) Background Art
A storage-type lean NOx hap (LNT) catalyst reduces emission of nitrogen oxides (NOx) by storing them in the form of nitrates under a lean burn condition. In the lean burn condition, reduction of NOx by a precious metal is difficult due to high oxygen content in an exhaust gas as compared to the theoretical air-fuel ratio at which a fuel can be completely burnt by air. After a prolonged time, when the NOx storage capacity of the catalyst reaches an upper limit, the oxygen content in the exhaust gas is decreased through post-injection control. Te content of reducing components, such as CO/HC, is then increased, such that the stored nitrates are desorbed and reduced to nitrogen (N2) by reducing agents (e.g., HC, CO, H2, etc.) as shown below:Reaction during NOx storage: BaCO3+2NO2+½O2→Ba(NO3)2+CO2  (1)Reaction during NOx reduction: Ba(NO3)2+2R→2NOx+BaO+2RO2.5-xNOx+R→½N2+ROx  (2)
(In (2), R denotes a reducing agent.)
The NOx storage occurs in a temperature range of 100-400° C. and the NOx reduction occurs at 250° C. or above. In the case of an LNT catalyst for a diesel engine, the NOx storage occurs at a temperature lower the above temperature range and, thus, cerium (Ce) is often used together with barium (Ba) to store NOx. Although cerium (Ce) has good storage capacity at low temperature as compared with barium (Ba), it has a smaller storage capacity than Ba. Hence, when the catalyst temperature rises abruptly due to acceleration of a vehicle, the cerium (Ce) fails to store the adsorbed NOx, and instead releases it (thermal desorption). This is the major cause of poor NOx purification.
The LNT catalyst was proposed in early 1990s by Toyota Motor Corporation, when it was developed as a lean-burn gasoline catalyst. In application, an additional three-way catalyst is disposed before the LNT catalyst. Since the catalytic reaction temperature is higher for a gasoline engine than a diesel engine, a catalyst containing as much as 10-20 wt % of a high-temperature storage material such as Ba, K, etc. was developed. Later, a catalyst further containing Ce for storage at low temperature was developed.
As a support of the LNT catalyst, alumina is mainly used. Korean Patent Application Publication No. 2009-0086517 describes NOx storage catalysts wherein a metal such as platinum, palladium, cobalt, etc. and barium are supported on a porous alumina support. Korean Patent Application Publication No. 2010-0061152 describes a NOx storage catalyst consisting of a diesel fuel cracking catalyst, a nitrogen oxide storage layer and a nitrogen reduction layer, wherein the nitrogen oxide storage layer includes a catalyst in which barium (Ba) is coated on an alumina support and the nitrogen reduction layer includes a catalyst in which platinum (Pt) is supported on an alumina-ceria (Al2O3—CeO2) support. However, supporting barium (Ba) on the alumina support leads to formation of BaAl2O4 as a result of reaction between barium and alumina. This undesirably decreases NOx storage capacity. In an attempt to solve this problem, use of a magnesium (Mg)-substituted spinel alumina (MgAl2O4) support was developed.
Due to the recently announced Euro VI emission standard, NOx reducing catalysts will be increasingly used in most diesel vehicles to meet the new standards. Further, because Euro VI lowered the allowed NOx emission by about half as compared to the prior Euro V emission standard, further improvements in the NOx reducing catalyst are necessary.